Control of constituent potentials



Feb. 13, 1951 BESSELMAN EIAL 2,541,357

CONTROL OF CONSTITUENT-POTENTIALS Filed May 50, 1945 2 Sheets-Sheet lFeb. 13, 1951 w. L. BESSELMAN ETAL 2,541,857

CONTROL OF CONSTITUENT-PQTENTIALS 2 Sheets-Sheet 2 Filed May 30, 1945 INVEN T 0R3 Tic 2. 3

#4041451 555554414 fiir /womo 1. 1241 4917 Patented Feb. 13, 19 51CONTROL OF CONSTITUENT POTENTIALS Wayne L. Besselman, Philadelphia, andRaymond L. Davis, lI, Mel-wood, Pa., assignors to Leeds and NorthrupCompany, Philadelphia, Pa., a corporation of Pennsylvania ApplicationMay 30, 1945, Serial No. 596,792

This invention relates to an apparatus for determining theconstituent-potential of gases, more particularly to the determinationof the constituent-potential with respect to work undergoing treatmentby gases such as in carburizing or nitriding of metals, and has for anobject-the provision of a system in which the constituentpotential maybe determined coincidentally with the treatment of the work.

Heretofore it has been proposed to measure carbon pressures orpotentials by utilizing a very fine carbon steel wire which is heated byelectric current flowing therethrough to a temperature of the order of800 C. to 1100 C. while subject to a stream of carburizing gases. Afterequilibrium has been established, the wire is rapidly cooled to producea martensitic structure. Thereafter the resistance of the wire ismeasured. It has been stated that there is correlation between theresulting resistance of the wire and the carbon content thereof,providing the wire upon cooling is completely in the martensitic phase.

Systems of the foregoing type have many disadvantages. They requireseparate treating chambers, and they do not permit continuousmeasurement of constituent-potentials. In consequence, they are notsuited for continuous automatic control of gaseous treating mediums.

The term carbon-potential and the broader term constituent-potential areherein used to denote a unit or factor which expresses the extent oftransference of carbon from a gas into work, or transference of carbonfrom the work to the gas. Since this same phenomenon is also present asbetween nitrogen and steel, and between other gases and metals, thebroader term "constituentpotential" is intended to denote thetransference and the possibility of transference of material between agaseous medium and the work subjected thereto, which transference variesthe composition of the case, the latter meaning the surface andnear-surface region affected by said transference.

In accordance with the invention, there is provided a system of not onlymeasuring the constituent-potential of an atmosphere but also one inwhich the said potential of the atmosphere may be continuously measuredand controlled and which by integration of all the variables of theprocess may be utilized to predeterminethe ultimate compositionalcharacteristic of work as affected by the atmosphere.

In carrying out the invention in one form thereof, a wire or filament ofiron, or an alloy thereof, is .disposed adjacent the work and both aresubjected to the same temperature and atmosphere. The variation in anelectrical characteristic, the resistance, of the wire or filament ismeasured and is utilized to indicate the constituent-potential of thegaseous medium or atmosphere.

12 Claims. (Cl. ass-2) For a more complete understanding of theinvention and for further objects and advantages thereof, referenceshould be had to the accompanying description, taken in conjunction withthe drawings, in which:

Fig. 1 diagrammatically illustrates the invention in one form as appliedto a carburizing furnace for the measurement and control ofconstituent-potentials;

Fig. 2 is a longitudinal view, partly in section of a part of thedetecting element 30 of Fig. 1;

Fig. 2-A' is a sectional view taken on the line iA-IA of Fig. 2;

Fig. 2-B is a sectional view taken on the line 18-23 of Fig. 2;

Fig. 3 is an alternative arrangement for the measurement of control ofconstituent-potentials;

Fig. 4 is a longitudinal view, partly in section, of a modified form ofdetecting element;

Figs. 5 and 6 are sectional views taken respectively on the lines 5-5and 8-8 of Fig. 4; and Fig. 7 is a fractional side elevation, partly insection, of Fig. 4.

Referring to the drawings, the invention in one form has beenillustrated as applied to a carburizing furnace it of the type disclosedin Harsch Patent No. 2.161.162. Though treating chambers or carburizingfurnaces of other construction may be utilized, the one disclosed insaid Harch patent has been found to be satisfactory. For a detaileddescription of the structure and the carburizing process in general,reference may be had to said patent. Briefly, the carburizing furnace l0consists of a reaction chamber C defined by a retort II and a cover l2,a seal i3 being provided between the cover and the retort. The work orload, though it may be a single piece, has been illustrated ascomprising a group or batch of pieces disposed haphazardly in a basketII. This basket is provided with a perforated plate or grid IS. A fanIt, driven by a motor i1, forcibly circulates through the load hotvehicle gases which serve as a carrier for the carburizing components orconstituents of the furnace atmosphere.

The carburizing agent, in liquid, gaseous, or vapor phase, is suppliedto the reaction chamber C by way of an inlet pipe 20, under the controlof a valve 2 I, and after passing through a dehydrator 22 is directedagainst a distributor plate 23. The hot vehicle gases swirl about in thespace below the cover and above the work and entrain and distribute thefreshly introduced agent quickly to bring it to dissociationtemperature.

The work pieces and the vehicle gases are heated to,carburizingtemperature, usually in the neighborhood of about 1700 F. in anysuitable manner, such, for example, as by electrically heated resistors25 disposed in the heating chamher 2 which surrounds the retort H. Thecar- I burizing agent is quickly heated to a temperature to causecracking or dissociation thereof. to yield an atmosphere having thedesired constituent or carbon-potential with respect to the steel orferrous metal parts of the load. It is important to the Harsch system ofcarburizing that the carburizing agent shall be introduced into thecarrier gas at a point such that the agent reaches the crackingtemperature as it approaches the work. In consequence, the carburizingaction takes place with the reaction products in their nascent state.

It is to be understood the present invention is applicable to systemsother than as shown by said Harsch patent, for example, to systems wherethe vcarburizing agent is given a preliminary or final cracking outsideof the reaction chamber.

In accordance with the invention, the constituent or carbon-potential ofthe gaseous medium toward the work is determined by means of an element80 which includes a relatively small Per cent Carbon .08 Manganese .23Phosphorus -4 .017 Sulphur .043 Silicon --less than .003 Nickel .047Chromium less than... .01 Copper .123 Tin less than .01 Iron 90.46

This particular ferrous alloy is one which is used quite extensively inthermocouples, as described in Finch Patent No. 2,325,759. Thoughferrous alloys of widely differing composition may be utilized, the onespecifically disclosed is preferred because of its availability andbecause of the need to provide replaceable elements of the samecomposition. This wire Ii is connected at one end to a conductor 32 andextends downwardly through a porcelain or other refractory support 33,Figs. 2, 2-A and 2-H, around the end thereof and upwardly to a secondconductor 34. The ceramic rod I3 is provided with a series of elongatedrecesses or openings 88 which expose the wire to the furnace atmosphere.

A 12-inch length of the foregoing wire at a temperature of 77 1". has aresistance of 0.73 ohm. The element 30 is disposed in the reactionchamber C in a position to be subjected to the same gases andtemperature as the work. Both the wire II and the work arecoincidentally subiected to transference of material to and from thegases. Carbon is absorbed or given up by the wire Ii until equilibriumconditions are attained. These equilibrium conditions are rapidlyestablished because of the short carbon path through the wire orfilament ii. In accordance with the invention, it has been discoveredthat an electrical characteristic of the filament or wire ll, theresistance thereof. will vary to a degree dependent upon the amount ofcarbon or other constituent taken up or removed from the wire. Forexample, at a temperature of 1700 P. the resistance of the wire ll, with0.2% carbon therein, will be 6.95 oms; for the same temperature asimilar wire with a carbon content of 1.00% will have a resistance of7.21 ohms.

In the preferred form of the invention the sensitive element 30,including the wire 30, is at all times subiected to the same ambientconditions as that of the work, and coincidentally therewith. Inconsequence. not only is a determination of the character of theatmosphere possible but also there is provided a means for controllingor regulating the character of the atmosphere within the reactionchamber C. In accordance with the invention. the foregoing variations inthe resistance of the wire ii, at high temperature, occur withcompositional changes in the wire itself. Contrary to prior knowledgeand belief there are provided new and valuable methods and means ofcontrolling the atmosphere within the reaction chamber C. Stateddifferently, it has been discovered that the electrical resistance ofthe wire 3| varies though the wire has an austenitic structure; that is,with the carbon thereof largely in a free state, or in solid solution.The resistance, other factors remaining the same, is at all timesdependent upon the carbon present in the wire or filament II. It hasalso been discovered that with the wire Ii heated to temperatures ofl000-l 1'2, the

same property of varying resistance is exhibited with respect tonitrogen. Hence, the invention is applicable to nitriding purposes andthe like.

The simplified form of a measuring system illustrated in Fig. 1,consists of a network in the form of a Wheatstone bridge W in which onearm thereof comprises the iron wire 3| while a second arm thereoi.'includes a sealed element Iii having within the interior a suitablelength of wire having the same resistance-temperature coemcient as thewire 3!. It may be of the same material as wire 3 I, though wire orwires of differing material but with the same temperature-coemcient ofresistance may be used. It is sealed within the container 40 to isolateit from the atmosphere of the reaction chamber C. By connecting thisresistor in the second arm of the Wheatstone bridge, automaticcompensation is introduced therein for changes in resistance of the wireIi due solely to temperature changes. It has been found that theresistance of the wire 3i varies substantially linearly with temperaturefor temperatures above around 1400 I". The temperature coefllcient ofresistance for practically any carbon content, at least between 0.25% to1.25%, is 0.018% per degree F.

A third arm of the bridge W includes resistors ll and 42 and a slidewire43, while the fourth arm of the bridge includes resistors 04 and 45 andthe slidewire 40. A resistor I1 and a slidewire ll are provided betweenthe last-named arms of the bridge. A suitable source of current, as thebattery 00, is connected through a variable resistor Ii to the conjugatepoints 52 and 53 while a sensitive detecting instrument B4 is connectedbetween the conjugate points 65 and 66. The slidewires II and 48 aresimultaneously and 0ppositely adjustable and are preferably mechanicallyconnected together as illustrated by the broken line 51. By adjustingthe slidewires 43 and 40 to shift resistance from one arm of the bridgeW to the other, adiustment for variation in the circuit constants,particularly the cold resistance of element 30, may be made.

The detecting instrument 54 includes a sensitive galvanometer and amechanical relay preferably of the type fully disclosed in Squibb PatentNo. 1,935,732. Reference may be had to said Squibb patent for details ofthe galvanometer and mechanical relay. Reference may also be had to LanePatent No. 2,119,108 for disclosure of a mechanical relay systemsuitable for operation or control of the valve 2|.

Inasmuch as it has been discovered that the resistance of the wire 31,when in equilibrium with the atmosphere surrounding it, varies with thecarbon or constituent-potential of that atmosphere, the slidewire 48associated with the mechanical relay and the chart or scale thereof (notshown) may be calibrated directly in terms of carbon content in thesurface of a particular steel, or in terms of the carbon content of athin shim steel of composition, for example, S. A. E. 1010, known asshim steel. In Fig. 1, the scale 6i is illustrated and it will beassumed it is so calibrated. In operation, the slidewires 43' and 46will be preset to positions to calibrate the bridge W in terms of thecarbon content indicated by scale 61. The resistors 25, of the furnacei0, will be energized to produce the desired temperature, generallyaround 1700 F., within the reaction chamber C. Under the control of theinstrument II, the valve 2| will be opened to a degree which permits theentry into the chamber C of a sufficient quantity of a carburizingmedium which, upon contact with the wire 31, will add to or subtractcarbon from the wire until the composition thereof is at a selectedvalue. The instrument 54, through valve 2|, continues to control theatmosphere within the chamber C so as to maintain constant thecomposition of the wire 31. Accordingly, both a continuous measurementand control of the carbon or constituent-potential of the atmosphere areprovided.

The shaft ill of the Sqibb Patent 1,935,732 may directly control valve2| or it may indirectly control that valve. In either case, adiustingmeans 62 are provided to change the actuation of the valve with respectto the relative positions of the contact 59 and the slidewire 4B. Thisadjusting means in simple form may compr se a lengthening and shorteningmeans for the linkage indicated by the broken line 60. In this manner,any predetermined constituent-potential of the atmosphere withinreaction chamber C may be maintained.

A duration-type of control as disclosed in Davis Patent No. 2,325,232,particularly Fig. 3, has been found satisfactory. The slidewire 3thereof will comprise adjusting means 62, while slidewire 39 of thepatent will correspond with slidewire 48 of Fig. 1 hereof. By changingthe relative positions of the adjusting means 62 (slidewire 3 of thepatent) and slidewire 48 (slidewire 33 of the patent) thecarbon-potential may be preset or selected at any disired value. Thevalve I24 of the patent will, of course, be replaced by the valve 2| ofFig. 1 hereof.

In the foregoing description it is to be understood that in theoryneither the length of the filament 3| nor its cross-sectional area issignificant. It may be of any desired length. It may have any desiredcross-sectional area. However, in practicing the invention, the smallerthe lesser cross-sectional dimension is, the faster will equilibrium bereached with a given potential of I 6 istic, however, is preferably onewhich may be defined as including one dimension which is small for rapidequilibration of the constituent in the filament 3i and in the gaseousmedium. In the preferred forms of the invention, satisfactory operationhas been obtained with round wires varying in diameter from 0.003" to0.040". Ribbons of similar thicknesses are likewise satisfactory. In theforegoing example, the wire 30 had a diameter of 0.010.

In carrying out the invention in one form thereof, a plurality of workpieces of the same composition were placed in the reaction chamber C andthe controller was set for the maintenance or production in the case orsurface layers of the work pieces of 0.65% carbon, as by the adjustingmeans 62. The heating resistors 25 were then energized to maintain thereaction chamber at a temperature of 1450 F. At intervals during aperiod of approximately twenty-four hours samples were removed throughthe outlet or vent pipe 63. These samples, upon analysis, had a case,'ora surface'and subsurface composition of between 0.65% and 0.66% carbon.

The foregoing procedure with like work pieces within the reactionchamber C was then repeated for a second 24hour run, with thetemperature of the reaction chamber C at 1750" F. Again, work piecesremoved at intervals during this run showed a case between 0.64% and0.66% carbon. Though the temperature had been increased by 300", thecarbon content of the case of all work pieces was closely held betweenthe limits of 0.64% and 0.66 /2. The carbon content through out the24-hour runs was maintained fully as closely as quantitative analyses ofcarbon content can be made.

With like samples of work pieces. the adjusting means 62 was then setfor a carbon cont nt of 0.50%. For one run the temperature of thereaction chamber C was maintained at 1450 F. and for another run it wasmaintained at 1750 F. All samples withdrawn at intervals during each runshowed a carbon content within the range of 0.47% to 0.52%. Additionaltests were made, with like workpieces or samples, for runs respectivelyat 1450" F. and 1750 F. but with the adjusting means 62 set for a carboncontent of 0.85%. Again, the samples removed at intervals during eachrun showed a carbon content which ranged from 0.85% to 0.90%.

The foregoing exemplary tests demonstrate an accuracy of control whichhas heretofore been unattainable and by means of which accurate controlof carburizino, at different temperatures has been made possible.

It is not essential that a second tube element 40 be provided in asealed container. In ac corcianee with a further preferred modificationof the invention, Fig. 3, automatic compensation for changes in thetemperature of the reaction chamber C may be provided by means of adetectin'z instrument 65 which is preferably of the type di clos d inthe aforesaid Squibb Patent No. 1,935,732. The instrument 65 operates inresponse to the temperature of a thermocouple 66 disposed within thechamber C and not only serves relatively to ad-ust a slidewire 67 withrerpect to its contact 68 but also to effect, through mechanicalconnection 65a, relative adjustment between a slidewire 69 and itscontact 10. The slidewire 61 in conjunction with a source of supply suchas a cell H and resistor Ha forms a potentiometer which, under thecontrol of the device 65, provides a potential to balance that producedby the thermocouple 00. The slidewire and its associated resistor I!introduce a correction into the' Wheatstone bridge W to correct for thevariation with temperature of the resistance of the filamentary metal orwire il, that is, the combined resistance of resistor 12 and slidewire uis so varied as to'compensate for the temperature component of theresistance variation of the filamentary metal or wire 0i. Hence. thenetwork W will respond onlyto the change in the compositional componentof the resistance variation of said filamentary metal or 8 equilibriumis attained, the carbon content at thesurface and throughout the wire llwill not change as long as the given conditions are maintained. Thedepth of the case on the work pieces will vary as a function of time andtemperature but the carbon content at the surface and subsurface will bemaintained as above described.

The same principles may be likewise applied to other processes. Forexample, a wire of iron or of an iron alloy when exposed to anitrogen-bear-- ing atmosphere will gain or lose nitrogen. As in thecase of carbon. there will be a nitrogen or constituent-potential asbetween the nitrogen and the work which in general consists of alloys ofthe; type known to those skilled in the art as "Nitralloy." As appliedto a nitriding process.

and utilizing a wire II of the same character as above described. theresistance of the wire ll, varied through an even greater range thanfor,

there has been substituted for the element 40 of Fig. l, a fixedresistor 40a. Further variation, in accordance with Fig. 8, is theprovision in supply line 20 of a control valve Ila which is operated bya solenoid ll under the control of contacts ll. Whenever the instrumentI detects a deviation in the carbon content it will be understood thatan adjustment is made between the slidewire 50 and its associatedcontact 00. It will be assumed the adjusting means 02 is set for acarbon content of 0.75% which value will be assumed to produce balanceof network W with the contact as in the position illustrated in Fig. 3.If the carbon content should be less. the unbalance of the network willcause the contact I! to move to the left with respect to the scale II(or the scale and slidewire to move to the right). This movement will,through connection 10 and adjusting means 02, close contacts 10 toenergize solenoid 14, from source ll, thereby to open the valve Ila.Accordingly the flow of carburizing material to the chamber 0 will beincreased. As the carbon content in wire 8| rises to the predeterminedvalue of 0.75%, balance of network W will be re-established with theparts again in the illustrated positions. Should the carbon content riseto above the selected value. the contacts I! will remain open. The valve2 in will remain in a more or less closed position. Because of airinfiltration, the carburizing atmosphere will be diluted so as todecrease the carbon or constituent-potential thereby to decrease thecarbon content of the work pieces. If more positive control is desired,a diluent such as nitrogen may be introduced under control of mechanicalconnection 10.

The carburizing fluid introduced into the furnace by way of pipe 20 andvalve lio may be of the type disclosed in the aforesaid Harsch PatentNo. 2,161,162. It is to be further understood that the concentration ofcarbon in the surface of a metal quickly assumes an equilibrium value,notwithstanding the fact the carbon will continue to be transferredthrough the surface layer in order to establish a like concentration inthe subsurface or interior of the metal. By utilizing a filamentarymeans having one cross-sectional dimension small, this equilibrium isatcarbon.

As between carburizing and nitriding. one of the principal differencesis that in carburizing a" certain amount of air-infiltration, ispermitted.

though separate streams of air, steam, or a decarburizing medium may beutilized when desired.

In contrast, during nitriding the reaction cham-- her is ordinarilysealed to exclude air infiltration.

To dilute the nitriding atmosphere a secondstream of an inert gas suchas nitrogen may be utilized. The nitriding gas itself is preferablyammonia.

The sensitive element 00 may also take the form illustrated in Figs.4-7. As shown, an outer tubular housing 00, of a chromium-nickel alloyof the type known under the trade name "Chro-.

max," is provided at its lower end with openings Ii and If to expose tothe furnace atmosphere the wire Ii which is wound on a ceramic corecollar or washer 01 secured to an inner support,

00 of a ceramic material known as "Transite" by means of screws 00 and00. The block itself is held in place by a screw 0] which extendsthrough the housing 00. The lower end of the spring It bears against awasher 02 supported on a pin 08 carried by the plunger 05. Downwardmovement of the washer 02 and spring is limited by engagement of the pin00 or washer 02 with theshoulder 04 provided on the support 00.

The lower end 00a of the plunger is of reduced size and passes throughan opening in a guiding block 00. This block has a lower end 98a ofreduced size which extends into a plate 91 welded at its ends to thehousing 80, as best shown in Fig. 7. The lower end of plunger bearsagainst a contact plug I00 to which the upper end of wire II isfastened, as by welding. The pressure of spring 00 is applied fromplunger 00 to plug I00 to insure a good electrical contact.

From the bottom closure l0i, two studs I02 and I00, Figs. 4 and 6.extend into complementary openings provided in the ceramic core 84.These studs hold the core 84 in the illustrated position The upper endof the stud I02 bears against a tained without undue delay. After theforegoing 1 presses against stud I02 to make a good electrical contacttherewith. Hence, one of the conductors of network W leading to wire 3|may be attached to plunger 85, as by the screw I06, while the otherconductor may be attached to the housing 80. as at the screw 9 I, orotherwise. The upper end of the housing 80 is provided with a screwcapor closure I01 and a sealing gasket I08.

To remove the sensitive unit comprising wire 3| and support 84, the bodyportion is moved upwardly against the bias of the spring 86 until thelower end clears or is free of the studs Hi2 and The support 84 is thenmoved outwardly through one or the other of openings 8| and 82. Thesame. or another unit may be inserted in the supporting means bycarrying out the described procedure in reverse order. 1

As shown, the opposite ends of the core 84 are provided with slots H andIII which extend from one side of the core inwardly for communicationwith the end-recesses thereof. These slots H0 and III also communicatewith the recesses of smaller diameter into which the contact plugs I00and M4 are disposed. If after the assembly as a whole has been removedit is desired to replace the wire 3|, this may be accomplished byunhooking the wire from the pins H3 and H4. Enough slack must beprovided to permit the withdrawal of the plug I04 and for the removal ofthe wire through the slot Ill. After the wire has been unwound from thecore 84, it, and the contact plug Hill, are then lifted upwardly throughthe slot IIO. A new wire may be wound in place by repeating theforegoing steps in the reverse order. New elements do not need to befrequently provided, particularly if care is taken in withdrawing theelement from the reaction chamber C. To this end, the element maypreferably enter the reaction chamber C through an exit pipe, similar topipe 63, Fig. 1. By withdrawing it slowly through carbonaceous gases inthe exit pipe, they not only cool the element, including the wire 3|,but also prevent oxidation which would otherwise render it useless forlater use. In the absence of carbonaceous gases, the element is rapidlywithdrawn quickly to cool the wire 3!, thereby to minimize oxidationthereof.

In summary, there has been provided a continuous control system, basedon measurements made continuously during operation of the process. Thismeans that through the period when the temperature and the atmosphere issuch as to affect the work, the element will be affected coincidentallytherewith. There has, therefore, been provided a means for processingwork at any temperature practical for the process. protection of thework during cooling in the furnace before discharge therefrom. Upon thislatter aspect, by controlling the atmosphere, the work may be protectedduring cooling from loss of the material previously added thereto.

Though the principles of the invention have been fully set forth above,the following additional information is set forth as a further guide tothose skilled in the art. Though these data are presented in connectionwith a sensitive element of iron, 0. term used to include materialslargely composed of iron and which also includes carbon steels and somelow alloy steels, the invention may also be applied to other ferrousmaterials with respect to which a constituent-potential exists asbetween a treating atmosphere and work exposed thereto.

The system also makes possible the 10 In the following Table I there iscomprehensively set forth the relationship between the resistance of aone-foot length of the wire 3| and the carbon content of a particularsteel at four different temperatures:

TABLE I Carboncontent of Shim Steel (.3. A. E. 1010) vs. elementresistance Element [1450 o 11550 11650 50 F.

Resistance, Ohms per cent carbon In Table II there are set forth therespective carbon contents in the steel of Table I and in the surfacelayers of eight different steels at four different temperatures, thesteels being designated A, B G inclusive, and whose compositions, aswell as the steel of Table I, are given in Table III.

TABLE II Carbon contents of steels, per cent 3505" A n c 1) 10 F o alloyof the assassr TABLE III Composition of alloy steels of Table I! in percent 1 Shim Steel Drill Rod rm Solar GAE-621w GAE-M s l-nn s-u ass-10m ss o n a r o Oarbon 0.06015 0.00-1.00 0.90 0.50 096-1.!0 0.10-0.11 0.17010-0.!)

gen 0.30-0.60 0.1! 1.26 0.40 0.N-0.B0 0.30-0.50 (LN-0.60 0.40-0.70Nickel 4.10-5.95 ass-s. 1s 1. ss-aoo Chrome- 0.80 1. $4.00 1.25-1.75ran," an coo-Mo Tn 0.60 llllmn 1. 00

l Remainder iron.

By referring to Tables I and II, it will be readily apparent that if acase on the work is to have a carbon content of a particular value at aselected temperature, this carbon content is determined with respect tothe particular steel and the corresponding carbon content of the shimsteel.

A substantial part of the research work underlying this invention wasconducted on "Shim Steel" (GAE-i010). Typical am. are presented in TableI. The results, as shown in Table I. were then correlated in terms oftests made on other steels, as shown in Table II.

If it is desired to produce a case, or carbon content of approximately100 points, one per cent, in the surface layer of the steel D at 1750"1" reference will first be made to Table II. It will be noted that acarbon content of 0.988% will be attained under those conditions whichwill give a carbon content of 0.90% in the Shim Btee Reference is nowmade to Table I. The value 0.895% at 1750' I". will be taken asapproximately the 0.90% value. The resistance of the element II for thevalue 0.895% will be 7.3 ohms at 1750'1".

It will be remembered that the scale Ii, Figs. 1 and 3. is calibrated interms of the carbon content of the shim steel. Hence, the slidewire 4|,Pig. 1, or Bl, Fig. 3, may be set directly to correspond with the carboncontent 0.90%. The temperature of the chamber C is then maintained at1750' I". for time determined by the depth of the ease desired. A casewill then be produced having aoarbon content closely approximating thedesired value of one per cent. The carbon content will be predeterminedto within the accuracies now attainable with known methods ofquantitative analyses. If the slidewires 40 and II are not calibrated interms of carbon content. the resistance values of Table I may bereferred to.

It will be recalled that the resistance of the wire Ii variessubstantially linearly with temperatures above around 1400 F. This isabove the critical temperature of the wire Ii Further, in accordancewith the invention, the carbon content or constituent-potential may becontinuously determined for operating temperatures below approximately1400' I". by utilizing alloys of iron having lower critical temperaturesthan the example above set forth. More specifically, for the lowertemperatures. the wire I may consist of an following composition: nickle1.654%. molybdenum 03-03%, manganese 04-03%, carbon 04-02%, with theremainder iron. With this alloy. the critical temperature will be 50lower and therefore may be used for operations at temperatures around1850' I".

In accordance with another aspect of the invention, it has beendiscovered that in the reducing atmosphere which exists in the treatingchamber the contact elements I" and "a and the associated contacts inand I04 weld together. Upon initial installation of a sensitive unit 30the parts will of course be clean and the contact resistance will below. Because of the carbon-laden atmosphere one skilled in the art mightexpect that the contact resistancemight change during operation. Becauseof the aforesaid phenomenon the contact resistance does not change. Itremains constant and the union in the nature of a weld between thecontacts not only assures continuing stability in the resistance of thecontacts,

but it also prevents change in the resistance due to vibration of thesensitive element It including those contacts.

While preferred embodiments of the invention have been described, itwill be understood that further modifications may be'made withoutdeparting from the spirit and scope of the invention asset forth in theappended claims.

What is claimed is:

i. In a high temperature metal-treating system in which work issubjected to a gaseous medium having a constituent which gives rise to aconstituent-potential towards the work, means for determining saidconstituent-potential comprising a filamentary ferrous metal so disposedwith respect to said work as to be subject to the same temperature andgaseous treating conditions thereof. and having a composition such thatsaid constituent-potential with respect thereto is related to saidconstituent-potential towards said work, said filamentary metal havingat least one cross-sectional dimension of a size which produces rapidequalization of said constituent-potential by change in the compositionthereof, a balanceable network including means for connecting saidfilamentary metal in one branch thereof, a second filamentary metal,means for connecting it in a second branch of said network, means forsealing said second filamentary metal from said medium and forsupporting it in heat-transfer relation with said medium, and meansoperable solely in accordance with change in the electrical resistanceof said first-named filamentary metal due to compositional changesthereof for unbalancing said network for indicating during the treatmentof work by said gaseous medium of said constituent-potential withrespect to said work.

2. In a metal-treating system in which work is subjected to a gaseousatmosphere having at least one constituent which during treatment of thework is capable of transference to and from the medium and the work. ameasuring network including a filamentary ferrous metal disposed withinsaid atmosphere and whose resistance varies with its temperature andwith the composition of said atmosphere and a resistor in said networkwhose resistance varies solely with the temperature of said atmosphereto compensate for the temperature-component of the resistance variationof said filamentary metal, whereby said network may 'respond only to thechange in the compositional component of said filamentary metal.

3. In a high temperature metal-treating system in which work issubjected to a gaseous medium having a constituent which give rise to aconstituent-potential towards the work, means for determining saidconstituent-potential comprising a filamentary ferrous metal so disposedwith respect to said work as to be subject to the same temperature andgaseous treating conditions thereof and having a composition such thatsaid constituent-potential with respect thereto is related to saidconstituent-potential toward said work, said filamentary metal having atleast one cross-sectional dimension of a size which produces rapidequalization of said constituent-potential by change in the compositionthereof, a network including means for connecting said filamentary metalin one branch thereof, a resistor connected in a second branch thereof,means responsive to the temperature of said gaseous medium for varyingthe resistance of said resistor to compensate for change in theelectrical resistance of said filamentary metal due solely totemperature changes thereof, and means operable by unbalance of saidnetwork due to compositional changes in said filamentary metal forindicating the value of said constituentpotential.

4. In a metal-treating system in which ferrous work is subjected to agaseous medium having at least one constituent which during hightemperature treatment of the work is capable of transference between themedium and the work and which gives rise to aconstituent-potentialtherebetween, the combination of a filamentaryferrous metal whose resistance changes with transference in eitherdirection of said constituent between said filamentary metal and saidmedium, means supporting said filamentary metal for exposure to the sametemperature and constituent-transferring conditions as said work forvariation of its resistance in one direction on the other concurrentlywith constituent-transfer between said medium and said work, abalanceable network including means for connecting said filamentarymetal therein, variation in the resistance of said filamentary metalproducing unbalance of said network in one direction or the other, andmeans operable in accordance with said unbalance for measurement of saidconstituent-potential.

5. In a metal-treating system in which ferrous work is subjected to agaseous medium having at least one constituent which during hightemperature treatment of the work is capable of reversible transferencebetween the indium and the work and which gives rise to aconstituentpotential therebetween, the combination of a filamentaryferrous metal whose resistance changes with transference in eitherdirection of said constituent between said filamentary metal and saidmedium, said filamentary metal having a cross-sectional characteristicfor rapid equilibration of said constituent in said filamentary metalwith that in said medium by change in the amount of said constituent ineither direction rection or the other, and means operable in accordancewith said unbalance for measurement of said constituent-potential.

6. In a nitriding system in which ferrous work is subjected to anitriding atmosphere at a nitriding temperature having at least oneconstituent capable at said temperature of transference of nitrogenbetween the nitriding atmosphere and the work and which gives rise to anitrogenp0- tential therebetween to form a case, the combination of afilamentary ferrous metal whose current-varying resistance changes in acorresponding direction with transference in either direction ofnitrogen between said filamentary metal and said atmosphere, meanssupporting said filamentary metal for exposure to the same temperatureand nitriding atmosphere as said work for variation of its resistanceconcurrently with nitrogen-transfer between said atmosphere and saidwork, means for passing current through said filamentary metal, themagnitude of the current fiow varying with change in the resistance ofsaid filamentary metal in one direction or the' other due to change insaid filamentary metal in one direction or the other-of the nitrogentherein, and means operable in accordance with said magnitude of saidcurrent fiow through said filamentary metal for measurement of thenitrogenpotential between said nitriding atmosphere and said filamentarymetal in determination of the nitrogen-content of said case.

7. In a metal-treating system in which ferrous work is subjected to agaseous medium having at least one constituent which during treatment 01the work is capable of reversible transference between the medium andthe work and which gives rise to a constituent-potential therebetween toform a case, the combination of a filamentary ferrous metal whosecurrent-varying resistance changes with transference in either directionof said constituent between said filamentary metal and said medium,means supporting said filamentary metal for exposure to the sametemperature and constituent-transferring conditions as said workforvariation of its resistance concurrently with constituent-transferbetween said work and said medium, a balanceable network including meansfor connecting said filamentary metal therein, a change in theresistance of said filamentary metal producing unbalance of saidnetwork, and means responsive to unbalance of said network for varyingsaid constituent-potential in a direction to maintain a predeterminedcomposition of said filamentary metal thereby to control the finalcomposition of said case with reference to said constituent.

8. Ina metal-treating system in which ferrous work is subjected to ahigh temperature carburizing atmosphere having at least onecarbon-imparting constituent which during treatment of the work iscapable of reversible transference of carbon between the medium and thework and which gives rise to a carbon-potential therebetween to form acase. the combination of an iron wire of small diameter whose resistancechanges with transference in either direction of carbon between saidwire and said atmosphere, means supporting said wire for exposure to thesame temperature and carbon-transferring conditions as said work forvariation of its resistance in one direction or the other concurrentlywith carbon-transfer between said work and said atmosphere in onedirection or the other, a measuring network balanced for a predeterminedcarbon-content of said wire and unbalanced upon variation of saidcarbon-content from said predetermined value, and means responsive tounbalance of said network for varying said carbonpotential in adirection to return the carboncontent of said iron wire to saidpredetermined value thereby to predetermine the final carboncontent ofthe case of the work being carburized.

9. In a metal-treating system in which work of a ferrous alloy issubjected to a high temperature nitriding atmosphere having at least oneconstituent which during treatment of the work is capable oftransference of nitrogen between the atmosphere and the work and whichgives rise to a nitrogen-potential therebetween to form a case, thecombination of an iron wire of small diameter whose electricalresistance changes with transference in either direction of nitrogenbetween said iron wire and said atmosphere, means supporting said ironwire for exposure to the same temperature and nitrogen-transferringconditions as said work for variation of its resistance in one directionor the other concurrently with change of said nitrogen potential of saidatmosphere in one direction or the other. a measuring network balancedfor a predetermined nitrogen-content of said wire and unbalanced uponvariation of said nitrogen-content from said predetermined value, andmeans responsive to unbalance of said network for varying saidnitrogen-potential in a direction to return the nitrogen-content of saidiron wire to said predetermined value thereby to control the finalnitrogen-content of said case of said work.

10. Means for maintaining the constituentpotential between a constituentof an atmosphere capable of reversible transference between theatmosphere and ferrous metal at high temperature at a predeterminedvalue, comprising a filament of ferrous metal whose electricalresistance at said temperature changes in one direction with change ofsaid constituent-potential in one direction and which changes in theopposite direction with change of said constituent-potential in theopposite direction due to transference of said constituent to vary thenet content thereof in said filament in one direction or the other,

means supporting said filament in said high-tem perature atmosphere andsubject to the constituent-potential thereof, a measuring circuitincluding means for connecting said ferrous filament therein, saidfilament having a cross-sectional characteristic for rapid attainmentupon change of said constituent-potential in either direction ofequilibrium between said constituent in said atmosphere and saidconstituent in said filament, variation in the constituent-content ofsaid ferrous filament producing a resultant change of electricalresistance thereof, and control means reversibly operable in accordancewith said change of electrical resistance in one direction or the otherto change said constituent-potential of said atmosphere to maintain itat a predetermined value.

11. For a metal-treating system in which work is subjected to a gaseousatmosphere having at least one constituent which during high-temperaturetreatment of the work is capable of reversible transference between themedium and a ferrous metal and which gives rise to aconstituent-potential therebetween, a control system comprising afilament of ferrous metal whose electrical resistance at said hightemperature changes in one direction or the other with transference inone direction or the other of said constituent between said filament andsaid medium to change the net content of said constituent in saidfilament, means supporting said filament in said high-temperatureatmosphere and subject to the constituent-potential thereof, saidferrous filament having a cross-sectional characteristic for rapidattainment. upon change of said con-- stituent-potential in eitherdirection, of equilibrium between said constituent in said atmosphereand said constituent in said ferrous filament, a balanceable networkincluding means for connecting said ferrous filament therein. variationin said electrical resistance of said ferrous filament with change ofconstituent-content therein in one direction or the other producing bythe resultant change of electrical resistance thereof unbalance of saidnetwork in one direction or the other, and control means operable inonedirectionor the other in accordance with the direction of said unbalancefor restoring said balance of said network.

12. A control system for metal-treating of the type in which work issubjected to a gaseous atmosphere having at least one constituent whichduring high-temperature treatment of the work is capable of reversibletransference between the medium and a ferrous metal and which gives riseto a constituent-potential therebetween, comprising a filament offerrous metal whose electrical resistance at said high temperaturechanges in one direction or the other with transference in one directionor the other of said constituent between said filament and said mediumto change the net content of said constituent in said niament, meanssupporting said filament in said high-temperature atmosphere and subjectto the constituent-potential thereof, said ferrous filament having across-sectional characteristic for rapid attainment, upon change of saidconstituent-potential in either direction, of equilibrium between saidconstituent in said atmosphere and said constituent in said ferrousfilament, a balanceable network including means for connnecting saidferrous filament therein, variation of the constituent-content in onedirection or the other of said ferrous filament producing by theresultant change of electrical resistance thereof unbalance of saidnetwork in one direction or the other. and control means operable inaccordance with said unbalance to change said constituent potem tial ofsaid atmosphere in direction and by an amount to restore balance of saidnetwork.

WAYNE L. BEBSELMAIN. RAYMOND L. DAVIS. 11.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Nuir er Name Date 1,555,677 La Blane "MM"--- Sept.29, 1926 1,599,180 McIlvaine "mun" Sept. 7, 1926 (Other references onfollowing page) Number Number OTHER REFERENCES "Alloys of Iron andCarbon, vol. II, 81500, 1st ed., 1937, McGraw-Hill Book 00., Inc., N. Y.0., pages 587-590. 5 "Alloys of Iron and Carbon, vol. II, Sisco, 1sted., 1937 (previously cited), pages 589 and 590.

Certificate of Correction Patent No. 2,541,857 February 13, 1951 WAYNEL. BESSELMAN ET AL.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows:

Column 4-, line 6, for ems read ohms; column 5, line 43, for Sqibb readSquibb; line 64, for disired read desired; column 10, Table II, thirdcolumn thereof, under the sub-heading 1650 F. for the numeral .929 read.939; same table, fifth column thereof, under the sub-heading 17 50 Ffor .853 read .863; columns 11 and 12, Table III, seventh columnthereof, second line from top, for 0.80-0.50 read 0.30-0.60; column 13,line 52, for direction on read direction or; line 65, for mdium readmedium; line 74, after in, second occurrence, insert either directionof; line 75, strike out either direction; column 14, line 1, strike outof;

and that the said Letters Patent should be read as corrected above, sothat the same may conform to the record of the case in the PatentOtfice.

Signed and sealed this 28th day of August, A. D. 1951.

[SEAL] THOMAS F. MURPHY,

Assistant Gammissioner of Patents.

Certificate of Correction Patent No. 2,55,857' February 13, 1951 WAYNEL. BESSELMAN ET AL.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows:

Column 4, line 6, for oms read ohms; column 5, line 43, for Sqibb readSquib-Z); line 64, for disired read desred; column 10, Table II, thirdcolumn thereof, under the sub-heading 1650 F. for the numeral .929 read.939; same table, fifth column thereof, under the sub-heading 1750 F.for .853 read .863; columns 11 and 12, Table III, seventh columnthereof, second line from top, for O.30-O.50 read 0.30-0.60; column 13,line 52, for direction on read direction or; line 65, for mdiuln readmedium; line 74, after in, second occurrence, insert either directionof; line 75, strike out either direction; column 14, line 1, strike outof;

and that the said Letters Patent should be read as corrected above, sothat the same may conform to the record of the case in the PatentOflice.

Signed and sealed this 28th day of August, A. D. 1951.

[SEAL] THOMAS F. MURPHY,

Assistant Oommz'ssz'oner of Patents.

